The present invention relates to a process for a microwave oven for controlling a cooking operation as well as a microwave oven for implementing such a process.
A microwave oven typically comprises an oven cavity, a microwave source, a feeding system for feeding microwaves to the cavity, a control unit for controlling the power level and the time during which the microwaves are fed to the oven cavity, and means connected to the control unit for setting and/or calculating a cooking time for a solid or liquid foodstuff.
Various technologies have been proposed for sensor-controlled automation of the cooking operation, for example as described in U.S. Pat. No. 4,791,263 and our own Swedish Patent 9602159-7 (SE-C2-506.605).
However, the usability of these prior technologies is limited, since it is desirable that the user of the oven should be able to intervene during the actual cooking operation for various reasons.
The object of the present invention is to enable sensor controlled automatic cooking of a foodstuff utilising high powerxe2x80x94and, therefore, a short cooking timexe2x80x94and with full control even when there are user-initiated interruptions to the cooking operation.
It is a special object to enable such cooking in cases where the user first brings a cooking liquid to the boil and then places foodstuffs in the cooking liquid in order to cook them by boiling them.
The above objects are achieved according to the invention by a process and a microwave oven exhibiting the features stated in the appended claims.
The invention is thus based on the insight that special measures should be taken concerning the utilisation of sensor signals for control in connection with transitional phenomena which may occur when the user initiates an interruption in the cooking process, which interruption results in a hold time, during which the sensor signals may change in such an unfavourable way that there is a risk that the result of the automatic cooking process will be incorrect.
According to the invention, such a situation is handled by inhibiting the sensor control action for a period of time in connection with said hold time.
The invention is particularly suitable when said sensor is a moisture sensor, in which case there is a risk that the output signal from the sensor will be incorrectly high from a control point of view in connection with an interruption as a result of an uncontrolled moisture increase in the oven cavity, for example because the user lifts a lid placed on the vessel in which cooking is in progress.
According to the invention, it is advantageous if the heating before the hold time caused by the interruption as well as the continued heating until sensor-detected boiling takes place at high, preferably full, or at least essentially full power, after which simmering or boiling takes place during a certain cooking time. By inhibiting the sensor action during at least a certain period of time in connection with the continued heating one ensures that the simmering/boiling will begin at the right time.
According to the invention it is thus preferable for the heating to take place in a number of heating steps, comprising a first step before the interruption hold time, when, for example, the user places foodstuffs in at least essentially boiling cooking liquid, a second step when the liquid is brought to the boil again, and a third step when program-controlled simmering/boiling takes place. In this connection, the second step preferably begins with said time period when the sensor control is inhibited. When said time period has come to an end, the oven conditions have stabilised subsequent to the interruption hold time, so that the sensor output signal can safely be used for controlling the time of the transition from the second to the third heating step.
The length of the first heating step is advantageously also sensor controlled such that a sensor detects when the liquid begins to boil, whereupon the heating is interrupted and an output signal is given to the user, so that he can take the necessary measures during the resulting hold time.
Although it is possible to use separate sensors during the first heating step and the continued heating, according to the invention it is preferable to use the same sensor, which specifically can be a moisture sensor.
When a user takes some action during the interruption hold time, such as lifting the lid of the vessel and placing a foodstuff, e.g. pasta, in the boiling liquid, a large amount of additional moisture may accumulate in the oven cavity. When the oven is re-started the moisture will gradually be ventilated, but is has been found that there is a considerable risk that the moisture sensor will give an incorrect signal in connection with a restart, which signal prematurely indicates that boiling has resumed and, consequently, the cooking liquid will not be brought to the boil as desired. This problem is obviated by the invention, whereby, for a certain time subsequent to a re-start, the control system of the oven becomes insensitive to the moisture level inside the oven cavity. In this connection, high, preferably full, microwave power can preferably be applied also during the above-mentioned time period.
It has been found that because it can be expected that the time required to bring the cooking liquid to the boil is a function of the amount of the liquid and the time required for efficient ventilation of the moisture subsequent to a re-start can also be expected to be a function of the amount of the liquid, said time period of inhibited sensor action can advantageously be based on the time required to bring the liquid to the boil during the first heating step. The length of the time period can thus preferably be set to a certain fraction of the time required to bring the liquid to the boil, typically said time divided by about 16. As the person skilled in the art will readily appreciate, the time required can easily be determined by the control unit of the oven. Suitably, the actual heating time, taking into consideration any interruptions in the microwave feed, is utilised for this purpose.
As an alternative and/or a complement, the length of said time period could be related to the amount or character of the foodstuff, about which information can be inputted to the control unit of the oven by the user in the usual manner.
It has been found suitable to choose the length of said time period to be at least a predetermined minimum time, which typically can be about 20 seconds.
According to the invention, in connection with the first heating step, it is suitable to indicate to the user that the liquid is beginning to boil, after which the continued heating subsequent to the interruption hold time does not start until initiated by the user.
The third heating step of simmering/boiling during a cooking time advantageously takes place in a closed loop based on feedback from a sensor, which is preferably the same sensor that controls the initiation of the third heating step.
The cooking time of the third heating step can be set, for example by the time or food type inputted by the user, but it is, of course, also possible to relate the cooking time of the third step to sensor information obtained during the cooking operation, specifically the length of the first heating step and/or the length of the second heating step.
The third heating step, as well as the rest of the cooking operation where applicable, is suitably implemented according to the description in our above-mentioned Swedish patent 9602159-7, which is herewith incorporated by reference.